Identification card printers commonly utilize thermal printheads and thermal print ribbon to transfer dye from the ribbon to the card substrate to form an image thereon. The print ribbon includes different color frames or panels along its length. The frames or panels repeat in a sequence or group consisting of a yellow panel, followed by a magenta panel, which is followed by a cyan panel. In addition, black resin and overlay panels can be provided in the sequence of the color panels, if desired.
Ribbon sensors are used to detect the various panels of the print ribbon. Ribbon sensors typically include an emitter and a receiver that are positioned on opposite sides of the ribbon. The light received by the receiver is analyzed to determine the color of the panel being sensed by the sensor.
Prior art ribbon sensors typically utilize emitters that include a single yellow light emitting diode (LED) to detect the color panels. The yellow LED produces light having a wavelength of approximately 587 nanometers. The receiver, in the form of a photodetector, has a broad visible light wavelength response. When the yellow light passes through the color dye panels it is partially blocked depending on the light wavelength blocking characteristics of the dye of the panel. For example, the cyan panel blocks more of the yellow light than the magenta panel, which blocks more of the yellow light than the yellow panel.
The output signal from the receiver varies in accordance with the light received through the panels. Accordingly, the light received by the receiver through each panel results in a different output signal. This variance in the output signal is used to determine the color of the panel being sensed by the sensor.
Unfortunately, the differences in the output signals for the passage of yellow light through some types of cyan and magenta panels can be very small, making it difficult to distinguish those panels from each other. This problem is exacerbated by the slightly different wavelength blocking characteristics of panels of print ribbons from different vendors.